<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" placeholder="半径" id="a">
<input type="text" placeholder="周长" id="b">
<input type="text" placeholder="面积" id="c">

<script>
    var p = Math.PI;




//    var a = prompt("请输入");
//    console.log(Math.pow(a,2) * p);
//    console.log(a * 2 * p);





        var a = prompt("请输入");
        var b1 = a * a * p;
        var c1 = a * 2 * p;



        document.querySelector("#a").value = a;
        document.querySelector("#b").value = b1;
        document.querySelector("#c").value = c1;
</script>
</body>
</html>